Wall Design Questions
I am planning on building a new house inland a bit from midcoast Maine (Zone 6) and have been pondering issues related to exterior wall design, taking into account what I have read here and elsewhere. I want a relatively high-R wall (~mid R-30’s) and have been thinking about the following designs.
Design 1: 2×6 wall with ½” drywall, R-23 Roxul, OSB/plywood, 2-3” Roxul Comfortboard, rainscreen (e.g., 1×3 strapping), siding.
Design 2: 2×6 wall with ½” drywall, 2” Roxul Comfortboard (inside the studs), R-23 Roxul (between studs), OSB/plywood, rainscreen, siding.
Design 3: 2×8 wall (overall) consisting of (2) 2×4 walls offset from one another, ½” drywall, R-15 Roxul between studs of inner 2×4 wall, 1” Roxul Comfortboard “between” the 2×4 walls, R-15 Roxul between studs of outer 2×4 wall, OSB/plywood, rainscreen, siding. I ‘cheated’ a little here, assuming ~8” of mineral wool can be compressed into the 7-1/2” cavity.
In terms of R-values, the designs provide (based on R-4 per inch of mineral wool): R-31-35 (depending upon how much exterior comfortboard is applied), R-31, and R-34 (if compressing mineral wool a bit doesn’t significantly compromise R-value).
Design 1 would come up a bit short in terms of OSB/plywood protection for Zone 6 if only 2” of Comfortboard was used but I wonder if this would be “OK” insofar as the mineral wool should allow the sheathing to dry more readily than rigid foam. If this is ‘risky’ even with the better drainage capability of the wool then 3” would be applied. It seems to me that use of mineral wool on the exterior might be a better way to go from an R-value as well, given recent discussions about cold-weather performance of rigid foams (esp. polyiso).
Design 2 brings the protection from thermal bridging inside the house (reducing interior square footage a bit) but alleviates the need for exterior insulation (including the need to add enough insulation to keep the sheathing dry). The R value suffers a bit relative to the other designs, although this could be worked a bit by substituting rigid foam (R-5+) for the Comfortboard.
Design 3 incorporates protection against thermal bridging into the interior of the wall and has a higher R-value than design 2. It encroaches upon the interior square footage to the same degree as the second design and also alleviates the need for exterior insulation (including the need to add enough insulation to keep the sheathing dry). Although the first 2 designs are probably similar in terms of cost, this option would be more expensive (although I don’t know by how much).
Input on the merits (or lack thereof) of the three designs would be greatly appreciated. Is there a “best” design of the three?